The present invention relates to a process for treating with phosphine gas bulk commodities infested with or susceptible to infestation by pests, whilst being stored or transported in closed spaces such as bulk containers, bulk storage or transportation vessels or bins, silos, shipholds, bulk transport railway or road trucks or the like, comprising controlled hydrolysing over a period of about 0.25 to 20 days a suitable hydrolysable metal phosphide so as to generate phosphine gas, causing or allowing the phosphine gas to spread through the entire bulk commodity, there to be maintained in a concentration and for a duration adapted to destroy such pests as are present in the bulk commodity and to apparatus for carrying out such a process.
In RSA Pat. No. 79/2263 (U.S. Pat. No. 4,215,508 Allen et al) a process and apparatus are described as set out in the aforegoing. According to that disclosure measured portions of hydrolysable metal phosphide pest control composition individually packed in sachets of moisture and gas pervious material or directly are enclosed separately in pockets provided in a long flexible strip of substantial non-hygroscopic and substantially moisture-free material, the pockets being pervious to water vapour and gas. The aforesaid strips containing the portions of pest control agent have become known as "bag blankets" and will be referred to as "bag blankets" in the following (irrespective of whether or not the phosphide composition is first packed into sachets before being inserted into pockets of the strips or whether the pockets are designed to hold the composition--be it in powder, granule, pressed or other form--directly). In the present context bulk commodities are in particular heaped commodities and in particular agricultural bulk commodities such as grain, any other seed commodities, e.g. beans, soya beans, peanuts, cocoa beans, coffee, also any of the aforegoing in processed bulk form such as flour, furthermore non-edible bulk commodities susceptible to pest infestation. The commodities may be of vegetable or animal origin and could for example include fishmeal, bonemeal or carcass meal. According to the abovementioned patent specification, where a silo, storage bin, bulk store, bulk container, shiphold or the like containing such bulk commodity is infested or suspected to be infested by pests, e.g. rodents, but in particular insect pests such as grainborers, weevils, moths or the like, the aforesaid bag blankets containing a predetermined amount of metal phosphide pest control agent are unfolded and spread out rapidly on the upper surface of the bulk commodity, and the container, silo, storage space, shiphold, vessel or the like is closed and sealed off as gastight as possible. The moisture which is normally always present inside such closed spaces then enters in the form of water vapour into the pockets and sachets containing the metal phosphide pest control composition, causing a controlled hydrolysis of the metal phosphide resulting in the release of poisonous phosphine gas. Phosphine gas has a relatively high diffusion rate and as a result can penetrate in an acceptable period of time throughout the entire bulk commodity right to the bottom region thereof, even if the bulk commodity is heaped to a level many meters high. After a greater or lesser period of time it is found that phosphine concentrations throughout the bulk commodity have reached a level sufficient to kill off the pests which are to be controlled, provided the said phosphine level is maintained for an adequate duration. In general within the phosphine concentration ranges applied in practice by those skilled in the art, the time required for complete killing of the pests to be controlled is approximately inversely proportional to the phosphine concentration employed at any given temperature. The rate at which phosphine gas is released from metal phosphide compositions depends on the nature of the metal phosphide, (usually aluminium phosphide or sometimes magnesium phosphide and in certain instances also calcium phosphide), the particle size of the metal phosphide, the manner in which the metal phosphide is compounded with a variety of additives known to those skilled in the art, whether or not the composition takes the form of a powder, granulate, pellets or tablets or is pressed into slabs with fibrous carrier substances. The release rate also depends on the availability of moisture, i.e. the ambient humidity and the rate at which such humidity permeates through the material of e.g. the bag blankets and the sachets and finally on the temperature of the environment. In practice the release is only completed in a period of from about 0.25 to 20 days and usually in from 1 to 5, e.g. in the case of AlP. 2 to 3 days or longer, e.g. up to 10 days at low temperatures. The lowest concentrations are only lethal if maintained for 3-4 weeks. Suitable phosphine gas concentrations for pest control purposes are in the range of from 50 ppm to 5000 ppm (parts per million), usually not less than 100 ppm, preferably 500 ppm to 2200 ppm, and the recommended periods of exposure of the bulk commodity to the aforesaid concentrations of phosphine gas for reliable disinfestation are from 500 hours to 48 hours and at the preferred concentration from 250 hours to 50 hours or from 250-100 hours for very resistant pests.
The above-described method of using "bag blankets" has proved to be highly effective in practice, resulting in complete disinfestations within acceptably short periods of time. The bag blankets can be introduced rapidly and without appreciable loss of phosphine gas or health risk to the operators and after completion of the fumigation the spent bag blankets can be easily retrieved and disposed of without leaving behind any residues of the pest control compositions in the bulk commodity. However, although the diffusion rate of phosphine gas is high, the period required for complete penetration to extreme distances from the locality of application of the bag blankets can be quite appreciable, e.g. 2 to 3 days. In very large silos the time taken by the gas to travel downwards may be too long to be acceptable, e.g. 2-3 weeks in extreme cases. Moreover, at any given period of time, different phosphine concentrations are likely to prevail in different parts of the bulk material. To ensure that all parts of the bulk commodity are adequately exposed to the gas, it is thus necessary to either employ relatively large quantities of metal phosphide composition or to employ excessively long exposure times. Long exposure times and long periods of travel of the phosphine gas through the bulk material are disadvantageous, because during such periods losses of phosphine gas may occur due to slow leakage and other causes known in the art.
Also during the diffusion of the phosphine gas downward, large amounts of phosphine gas simultaneously accumulate in the space above the bulk commodity, particularly if a storage or transport space is only partly filled, and in such empty space the phosphine gas is substantially ineffective and wasted. Finally, in order to supply adequate concentrations of phosphine gas to all parts of the bulk commodity, it may be unavoidable to have very much higher concentrations in the more immediate vicinity of the bag blankets, such concentrations may be higher than the optimum range. If rapid very high accumulations of phosphine gas occur, this may result in putting certain pests into a state of suspended animation or temporary coma without actually killing them, which may result in such pests surviving the fumigation period. Also, excessive accumulations of phosphine gas could in extreme cases result in flammable or even explosive mixtures of phosphine and air. For safety reasons this is obviously undesirable.
In South African patent No. 79/6807 (corresponding to U.S. Pat. No. 4,200,657) methods are described wherein a fumigant, in particular phosphine gas is caused to flow very slowly through a heaped bulk commodity contained in a silo, storage container or the like. According to the more specific disclosure, tablets of phosphine releasing metal phosphide composition are scattered over the surface of the bulk commodity and light suction is applied to the bottom of the bulk commodity pile, the air there withdrawn being circulated to the top of the container. According to that disclosure "the method has been successfully tested with an air flow low enough to effect a 3.5 day air change" while "optimum results have been obtained from the method when the rate of the air is maintained between approximately 0.0015 cubic feet per minute per bushel, (0.0014 m.sup.3 /min/m.sup.3 grain=6.5 hours air change) and 0.0008 cubic feet per minute per bushel (0.00075 m.sup.3 /min./m.sup.3 grain=11 hour air change)".
A less preferred method according to that disclosure, designed to achieve the same effect is to apply higher rates of flow for very brief durations, e.g. from 1 to 5 minutes spaced at intervals of 3 to 4 hours, so that for the flow rates in that case contemplated the ultimate result is equivalent to the aforegoing, namely a total air change over a prolonged period which may be as long as 3.5 days. However, such a procedure involves additional risks, particularly at high ambient temperatures at which the explosive limit of PH.sub.3 is reduced and at which PH.sub.3 becomes auto-ignitable even by relatively slight pressure changes. Whereas at 20.degree. C. the auto-ignition limit is at 17900 ppm this limit drops to lower values at higher temperatures. That limit can easily be attained after 6-8 hours if the circulation is too slow. However, even at ambient temperatures as low as 30.degree. C., the slow circulation can substantially increase the risk of auto-ignition. Scattering of tablets or pellets on the surface of the bulk commodities results in contamination thereof with the residue of the spent tablets or pellets.
The low rate of circulation does not avoid the aforementioned disadvantage that relatively large quantities of phosphine gas may accumulate almost ineffectively in the sometimes large empty space above the bulk commodity, nor will it avoid with certainty or at all the local accumulation of disadvantageously high concentrations of phosphine gas in the immediate vicinity of the phosphine generating composition. These prior teachings inter alia do not relate the circulation parameters to the rate of phosphine release. Accordingly, apart from the potential fire hazard, there is the risk of the aforesaid "narcosis effect" due to rapid excessive local concentration rises. On the other hand, elsewhere in the system the concentration rise will be too slow for optimally effective pest control. Moreover, a slow circulation rate is inclined to result in slow decomposition of the metal phosphide due to localized moisture starvation which is not replenished by moisture from elsewhere in the system.
The present invention is based on the recognition of the aforegoing disadvantages of the prior art and teaches ways and means for avoiding those disadvantages. More particularly, the present teachings can be employed to accelerate the rapid and sufficiently uniform distribution of phosphine gas throughout a bulk commodity, to improve fumigation efficiency both in terms of time and consumption of pest control composition and to avoid disadvantageously high local accumulations of phosphine gas. The invention can be employed to shorten fumigation times and to reduce losses of phosphine gas occurring when longer fumigation periods are employed and generally to optimise fumigation efficiency.